1. Field of the Invention
The present invention relates generally to systems and methods for producing low pressure plasmas. More particularly, it relates to equipment for producing highly uniform planar plasmas which can be used for treating semiconductor wafers in low pressure processing equipment.
Plasma generation is useful in a variety of semiconductor fabrication processes including etching, deposition, ion implantation and the like. Plasmas are generally produced in a low pressure gas by accelerating naturally occurring free electrons in the gas to the gas ionization energy, typically between five and twenty electron volts. Collisions between these energetic electrons and the gas molecules occasionally cause a molecule to be ionized, releasing an additional free electron. Each additional free electron is also accelerated and can also ionize gas molecules. The resultant partially ionized gas is called a plasma.
2. Description of the Prior Art
Most matter typically exists in one of four phases: solid, liquid, gas and plasma. Super hot plasmas are used in gas chromatographs to break down samples that have been injected so that they produce characteristic spectrums of light for analysis, and in semiconductor processing equipment to etch material away from a wafer. Plasma based semiconductor equipment also includes deposition processes, resist stripping and plasma enhanced chemical vapor deposition.
Larger and more uniform plasmas need to be generated now to keep pace with the ever increasing wafer sizes being processed. Whatever the step in the process, whether etching, depositing or implanting, the effects of the plasma typically need to be uniform across the entire surface. Present day technology includes eight inch wafer processing, and twelve inch wafer process fabrication facilities (fabs) are already being planned. The prior art equipment that was suitable for plasma etching two-inch wafers is unable to produce the uniformity that translates into maximum yields and better profits for state-of-the-art fabs.
Previous planar magnetically coupled plasma (PMCP) generating methods use a varying magnetic field generated by a planar spiral coil to generate the plasma, optionally combined with an independent radio frequency power source to accelerate the ions. This method is effective in providing independent control of flux and field with inexpensive equipment and simple operation through varying magnetic field electron acceleration. But it does not provide for uniform electron acceleration, especially near the axis of the coil. A prior art plasma etching system described by the present inventor in U.S. Pat. No. 4,948,458, issued Aug. 14, 1990, comprises a chamber with a dielectric window, such as quartz. A planar coil and capacitor combination is positioned adjacent to the window, with the axis of the coil perpendicular to the window, and a powerful radio frequency source is coupled to the coil. Power transfer is maximized by impedance matching and tuning to provide resonance at 13.56 MHz, for example. Inlet ports supply a process gas to the interior of the chamber. A varying magnetic field is induced by the coil into the process gas at power levels sufficient to create a plasma of the process gas that has a circulating flow of electrons. The electron motion is closely confined to a plane parallel to the planar coil, so transfer of kinetic energy in non-planar directions is minimal. The flow of electrons is planar, albeit annular, and therefore has an eye at the axis that has a lower energy level than the other parts of the planar plasma. The eye is unavoidable with a planar coil wound in a spiral, with the axis of the coil perpendicular to the plasma plane, as shown in FIGS. 1, 3, 6 and 7 of the patent.
A very common method for accelerating free electrons for plasmas in semiconductor wafer processing is to apply a radio frequency (RF) electric field between a pair of electrically conductive plates, or electrodes, on opposite sides of a process chamber that has been filled with a low pressure process gas. A wafer to be processed is typically mounted on one of these electrodes. As such, the RF electric field will accelerate electrons in the space between the electrodes with an ionization energy perpendicular to the surface of the wafer. The accelerated electrons will collide with process gas molecules, at a frequency dependent on the gas pressure. The collisions generate ions of gas which constitute the plasma. When the electrons strike the wafer, they are captured, and this results in a negative electric charge build up on the wafer. Such a charge will attract the positive ions circulating nearby in the plasma, and pull them to the wafer at high velocity. Whether the consequential ion impacts results in etching, deposition, or other effect, is a function of the type of gas generating the ions, the surface materials of the wafer, and other process conditions.
A single RF electric field will determine the levels of both the ion flux, which is the number of ions, and the ion field, which is the energy with which the ions strike the wafer. Independent control of the ion flux and field is therefore not easy to accomplish. However, some independent control of ion flux and field can be achieved by varying the gas pressure. As the gas pressure is lowered the distance between molecules increases, so electron collisions with molecules become less frequent. Since there are fewer collisions, there are fewer ions generated. Thus the ratio of ion flux to field is, in general, decreased. At pressures below approximately twenty pascals (0.15 torr), the ratio of ion flux to field becomes excessively low. Since pressures below twenty pascals are becoming increasingly important as semiconductor feature line widths decrease, the simple parallel plate method of plasma generation has recognized limitations. Several methods have been developed to circumvent the low pressure limitations of the parallel plate system.
In a magnetically enhanced plasma generation configuration, which is sometimes referred to as magnetically enhanced reactive ion etching (MERIE), a steady transverse magnetic field is used to curve the energized electron path. This increases the distance electrons must travel before they ultimately terminate in an electrode plate or wafer. While this method will increase the ion flux to field ratio, and permits a lower pressure for a given ratio, there are serious process disadvantages in having an intense transverse magnetic field so close to a wafer. This limits the lower pressure to approximately four pascals. U.S. Pat. Nos. 4,668,338, and 4,668,365, issued to Maydan, et al., and Foster, et al., respectively, describe a common approach to magnetically enhanced plasma generation.
Another prior art method, called electron cyclotron resonance (ECR), uses microwaves to accelerate the free electrons that create the plasma in a carefully controlled transverse magnetic field. The magnetic field is adjusted so that the rotation frequency of an electron, as deflected by the magnetic field, is equal to the microwave frequency. The electron energy is increased through a number of microwave power cycles until it reaches the ionization energy. For a microwave frequency of 2.45 GHz, the ECR magnetic field is 875 gauss. ECR plasma generation is often combined with a separate RF power source that is applied to an electrode on which the wafer is mounted. The separate source provides for ion acceleration. The microwave power therefore independently controls the ion flux and the RF power independently controls the ion field. While this method provides independent flux and field control, and can operate at low pressure, it also requires large and expensive magnets, combined with critical adjustments, for proper operation.
Other methods, in particular inductively coupled plasma (ICP) and helical inductor resonator (Helicon), are also used for plasma generation, but each has disadvantages in terms of efficiency, the ability to generate effective low pressure plasmas, and the ability to provide independent flux and field control. For more information on this subject, U.S. Pat. No. 4,421,690, describes an inductively coupled plasma (ICP) plasma generation apparatus, and U.S. Pat. No. 4,160,392, describes a helical inductive resonator plasma generation configuration.
Prior art plasma generation systems are not completely satisfactory in terms of their efficiency in generating a uniform planar plasma. Independent control of ion flux and field, in a simple, inexpensive configuration with no critical adjustments, is also lacking. While the planar magnetically coupled plasma system using a planar spiral coil is capable of satisfying most of these goals, there is a discontinuity in electron acceleration at the center of the coil that is an aberration in the desired plasma uniformity. However, there are advantages in providing transverse electron acceleration through the use of controlled amount of both electric and magnetic varying fields. At pressures below approximately ten pascals, an oscillating magnetic field is generally more efficient than an oscillating electric field in coupling energy to a plasma. However, an oscillating electric field may be needed to initiate the plasma. At pressures above approximately 100 pascals, an oscillating electric field is the more efficient.
An apparatus and method for generating highly uniform plasmas within semiconductor equipment is needed to support uses in etching, deposition and ion implantation equipment. Both electric and magnetic oscillating fields for acceleration of the ionizing electrons are desirable. The apparatus should preferably include a plasma generation capability that can generate high ion flux densities over a wide pressure range with a low ion energy directed toward a wafer being processed. An independent ion acceleration capability is needed for independent control of the ion flux and field to manage the ions striking the wafer. A simplicity of adjustment and operation, efficient operation in terms of power utilization, and small size are also goals that should be realized.
Therefore, an improvement in plasma generating technology is needed. The present invention overcomes the problems traditionally associated with plasma generation.